lubrication.. and drag. The specific transmission maker can speak to the former. Also, steering
is little goofy when hard over to compensate; less range of movement left, at least in one direction or the other.
Also, just because two mains are running, that doesn't necessarily mean twice as much fuel burn in the first place. If 300 horsepower (for example) is required to move a boat at or under it's theoretical hull speed
, that can come from one engine.. or from two, each providing about half of that 300 HP. The two smaller engines (or two engines running at lower RPMs) may burn closer to only 1.5x (or even less) than the single